1. Field of the Invention
The present invention relates to transparent electroconductive windows comprising a non-electroconductive assembly element, an electroconductive circuit portion carried by said element and one or more bus bars, adapted to be coupled to a source of electric potential to provide an electrical current through said circuit portion that is capable of heating the window. While the window may comprise a single assembly element in the form of a sheet of rigid, transparent material, the present invention has special utility in laminated windows.
Transparent electroconductive windows have been used in aircraft particularly for the purpose of removing fog, mist or ice that forms on a surface of the windshield when the window is exposed to extremely low outside temperatures while the temperature inside the aircraft is relatively warm. Heated windows of this type are also finding use in other vehicles, such as railway cars, boats, and custom automobiles, in other window structures such as buildings, and in transparent windows for freezers and the like. Essentially, the window comprises a base or substrate of rigid transparent non-electroconductive material, such as glass or a well known plastic substitute for glass, that supports an electroconductive coating which may be either a matrix of thin wires of thin strips of bus bar material or a continuous transparent electroconductive coating such as tin oxide and/or other metallic oxides or metals, which coating is in electroconductive contact with one or more bus bars. The latter are adapted to be connected to a source of potential whenever it is desired to apply electric current through the electroconductive coating so as to heat the window and dissipate a build-up of mist or ice that normally forms on a surface of the window under certain atmospheric conditions.
Since the transparent electroconductive coating is usually very thin and fragile, it has been the custom to protect it by laminating the coated substrate to additional sheets or layers of transparent material, using well known interlayer materials such as polyvinyl acetals, particularly plasticized polyvinyl butyral, and polyurethanes. Either or both other plies may be of glass or a transparent plastic.
In order to supply electrical energy for powering the transparent electroconductive coating, it is necessary that the coating be connected to a bus bar, which serves as a distributor of electrical energy from a voltage source to the electroconductive circuit portion. The bus bar must distribute the electrical energy uniformly to provide uniform heating over the entire coated surface. In addition, the electroconductive coatings usually used cannot withstand localized high currents without damage to or even destruction of the laminated window. When the coating is marred locally, an area of high current density may develop, which results in localized high heating.
It has also been suggested to use various materials other than a rigid transparent sheet to support the electroconductive circuit portion. For example, the thermoplastic interlayer material which is used to adhere a glass sheet or a sheet of other rigid transparent material to another sheet may itself serve as a carrier for narrow electroconductive wire. Also, a flexible interlayer may serve as the carrier for a transparent electroconductive circuit portion in the form of a coating bonded thereto. However, it is usually necessary that the circuit portion that provides the heat for the window be protected from scratching and other causes of damage, and also that the bus bar to heating element contact be maintained adequately throughout the entire area of contact between the bus bar and the electroconductive circuit portion of the heating element.
Regardless of the nature of the support provided in a window for the electroconductive circuit portion, the need existed for a suitable bus bar that is capable of making adequate contact with the circuit portion and of preventing or minimizing the effect of hot spots on the application of an electrical potential to said circuit portion.
2. Description of the Prior Art
Transparent electroconductive windows having ceramic silver bus bars making electrical contact with a transparent electroconductive coating are disclosed in many patents, including U.S. Pat. Nos. 2,7l0,900 to Linder and 2,724,658 to Lytle. In order to avoid bus bars which develop excessive stresses in the glass when the bus bars are applied to a glass sheet surface, the bus bar in these patents preferably should be located on the extremely edge of the glass and the bus bar thickness should be limited. This limitation as to bus bar thickness provides a limitation to the electroconductivity of the bus bar, hence such ceramic silver bus bars, while they are suitable for adhesion to glass, nevertheless, are extremely fragile and subject to burnouts, which cause disruptions and damage to the transparent electroconductive metal oxide coating on the glass sheet surface.
U.S. Pat. No. 2,813,960 to Egle et al and 3,440,408 to Brittan disclose laminated glass assemblies in which thin wire is used as the heating element. Despite these patents, a need still existed for a bus bar material that is relatively easy to apply and when applied would make a good, permanent electroconductive contact with the elongated wire elements carried by the interlayer material of a laminated window.
U.S. Pat. No. 3,223,829 to Davy et al discloses bus bars consisting essentially of a collodial dispersion of silver and an organic binding agent or cement in alcohol. These bus bars are disposed in contact with a series of heating wires. This material is baked to remove solvent and the material solidified to form a bus bar of silver and the organic binding agent of cement which is worked flat.
U.S. Pat. No. 3,249,466 to Lusher fuses a layer of solder glass containing finely divided paramagnetic material to glass.
U.S. Pat. No. 3,252,829 to Rumstadt et al discloses two bus bars with a lower fusion point than the glass substrate. The bus bars consist essentially of 55.6 percent by weight of silver, 7.8 percent by weight of flux, 18 percent by weight of organic binder and 8.5 percent thinner.
U.S. Pat. No. 3,469,015 to Warren discloses a bus bar of a silver paste consisting of silver flake and a binder resin disposed in a groove in a glass substrate.
U.S. Pat. No. 3,553,833 to Jochim et al discloses bus bars comprising silver strips attached to a glass base by soldering. The attached strips are a suspension of silver and lead borosilicate in an organic base which evaporates upon heating.
U.S. Pat. No. 3,623,906 to Akeyoshi et al discloses bus bars formed of solder applied using a soldering spatula whose tip is at 350.degree. C. during the bus bar application.
U.S. Pat. No. 3,705,047 to Marriott discloses the repair of broken electroconductive heating lines by applying a silver paste containing approximately 70 percent by weight of silver, 5 percent by weight of glass and 25 percent by weight of organic binders and solvents.
The above-identified patents either provide bus bars having a non-electroconductive component (organic binder or ceramic frit) which limits its electroconductivity or bus bars that are applied at elevated temperatures above those usually used in laminating and are not affected during exposure to glass laminating temperatures or bus bars that are applied in the solid state which limits the configuration of substrates to which the bus bars can be applied.